Electron beams are used in industry for welding of metals and for manufacturing alloys by selective melting. Ion beams are also used in industry for material modification by ion implantation, dry etching, and microfabrication. Such electron beam and ion beam manufacturing is typically performed in a vacuum since interaction between the charged particle beams and air molecules at atmospheric pressure causes rapid dispersion and attenuation of the beams which impairs their usefulness at relatively high pressures such as atmospheric pressure.
Electron beam welding has many well known advantages over other welding techniques including: very high depth-to-width ratio of the weldments; very high energy efficiency when electrical energy is converted directly into beam output energy; low distortions; and the ability to weld reasonably square butt joints without filler metal addition. Principle components of an electron beam welding column assembly include an electron gun, magnetic focusing coil, and a deflection coil contained in a vacuum housing or chamber. The electron gun must be held at a suitably low vacuum pressure below about 10.sup.-4 Torr. In order to effectively weld a workpiece, the workpiece is also typically contained in another chamber under vacuum. However, such vacuum welding of a workpiece results in relatively low production rates due to the required pumping time to suitably evacuate the chamber for each workpiece, and limits the practical size of a workpiece which may be contained in the vacuum chamber.
Non-vacuum electron beam welding may be practiced by maintaining the electron gun in a suitable vacuum, while providing a series of differentially pumped chambers between the electron gun and the workpiece which is maintained at atmospheric pressure. Although the pressure in each succeeding chamber increases for allowing the workpiece to be maintained at atmospheric pressure, the electron beam channeled through the chambers nevertheless experiences dispersion and attenuation which decrease its usefulness.
Material modification by ion implantation, dry etching, and micro-fabrication are commonly used technologies, all of which are necessarily performed in a vacuum since ion beams at energies used in these applications would otherwise be completely attenuated if passed through foil windows or through long differentially pumped sections such as would be required for maintaining the workpiece at atmospheric pressure.
Electron beam melting for manufacturing alloys is typically performed at a pressure of about 10.sup.-2 Torr. A major drawback of operating at this low pressure range is the loss of alloy elements with low vapor pressure. Accordingly, it is desirable to raise the operating pressure as high as possible and preferably as high as atmospheric pressure.